server: make the Accept trait public

[jmlemetayer]

Is your feature request related to a problem? Please describe.
Some external crates used the Server::run_incoming method to modify the behavior of the warp server (example: tokio-rustls-acme).

Now that this method has been replaced by Server::incoming and is no longer a Stream based argument but an acceptor, the server::accept::Accept trait must be public.

Describe the solution you'd like
Make the server::accept::Accept trait public.

Describe alternatives you've considered
N.A.

Additional context
See: n0-computer/tokio-rustls-acme#35

[seanmonstar]

I don't expect to make the trait public, at least not yet. An accept loop can be done outside of warp.

[jmlemetayer]

What do you mean by “accept loop”? I saw you write about it in this commit, but I don't understand how it works.

Perhaps adding an example would be helpful. Thanks